I think Exxon et al saying climate change is real and their own internal research confirms it would have changed policy. Better policies could have meant batteries, nuclear, solar, wind, etc could all have been cheaper faster. It could have meant a carbon border tax which would have hamstrung a lot of US manufacturing going to China. The negotiations between Obama and Lindsey Graham for a carbon price might have succeeded.

There's a whole "What if?" where the innovation and beginnings of a post-denial GOP could have happened earlier.